1. Field of the Invention
The present invention relates to a single-headed piston type swash plate compressor used for a vehicle air-conditioning system etc.
2. Description of the Related Art
A refrigeration circuit used in a vehicle air-conditioning system includes a compressor for compressing a refrigerant gas. This compressor comes in various forms such as variable displacement types and fixed displacement types. More specifically, fixed displacement type compressors include not only single-headed piston type swash plate types, but also double-headed piston type swash plate compressors. Variable displacement type compressors also include not only single-headed piston type swash plate compressors, but also double-headed piston type swash plate compressors.
Among these compressors, a general single-headed piston type swash plate compressor of the fixed displacement type or variable displacement type defines and forms inside its housing cylinder bores, a crank chamber, a compression chamber, a suction chamber, and a discharge chamber. Each cylinder bore accommodates a single-headed piston so that it may reciprocate. Further, a drive shaft supported rotatably by the housing is driven by an engine or another external drive source. The swash plate is supported to be able to synchronously rotate with respect to the drive shaft. A pairs of shoes is accommodated in a pair of shoe seats provided at an engagement portion in the piston, to drive the pistons and a shoe is provided at each of the front and rear of the swash plate.
Here, since each piston is a single-headed piston having a head at only one of the front and rear of the swash plate, the compressor is a single-headed piston type swash plate compressor. Further, if the swash plate is provided at a certain inclination angle with respect to the drive shaft, the compressor is a fixed displacement single-headed piston type swash plate compressor. If the swash plate is provided to be variable in the inclination angle with respect to the drive shaft and the pressure in the crank chamber can be adjusted by a control valve to change the inclination angle and adjust the discharge capacity, it is a variable displacement single-headed piston type swash plate compressor.
In this single-headed piston type swash plate compressor, if the drive shaft is driven by an external drive source, the swash plate synchronously rotates, so the pistons reciprocate in the cylinder bores. Due to this, each cylinder bore forms a compression chamber with the head of the piston, so when the piston is in the suction stroke, low pressure refrigerant gas is sucked into the compression chamber from the suction chamber connected to an evaporator of the refrigeration circuit. When the piston is in the compression stroke, high pressure refrigerant gas is discharged to the discharge chamber from the compression chamber. This discharge chamber is connected to a condenser of the refrigeration circuit. The refrigeration circuit is used as a vehicle air-conditioning system for air-conditioning a vehicle. During this time, in the swash plate compressor, the slidability of the sliding portions between the swash plate and the shoes is ensured by a mist-like lubrication oil contained in the refrigerant gas.
Japanese Unexamined Patent Publication (Kokai) No. 10-68380 discloses a variable capacity type single-headed piston type swash plate compressor having pistons formed by an aluminum-based material and having a swash plate formed by a ferrous material.
In this variable capacity type single-headed piston type swash plate compressor, since the material of the swash plate is made larger in specific gravity than that of the pistons, the centrifugal force of the swash plate acting in the direction reducing the inclination angle becomes larger. Therefore, in this variable capacity type single-headed piston type swash plate compressor, it is possible to prevent a decline in the high speed controllability due to the inertia of the pistons acting in a direction increasing the inclination angle.
In the above swash plate compressors of the above related art, however, whether of the fixed displacement type or the variable displacement type, due to the specific gravity of the shoes, the swash plate became easily worn under severe conditions and therefore the durability was not necessarily sufficient. In particular, when using a swash plate comprised of a swash plate substrate made of a ferrous metal and a coating of a nickel-boron plating etc. for improving the slidability formed on at least the piston side, that is, the rear surface, of the swash plate substrate, the coating easily becomes worn under severe conditions due to the specific gravity of the shoes and therefore the durability is not necessarily sufficient.
That is, in a single-headed piston type swash plate compressor, as shown in FIG. 7, the pair of shoes 92a, 92b slidability in the circumferential direction with respect to the swash plate 91. The shoe 92b provided at the rear side (right side FIG. 7) among the shoes 92a and 92b is pressed against the swash plate 91 by a load in accordance with the rotational angle. At this time, a differential pressure based on the difference between the pressure inside the compression chamber and the pressure inside the crank chamber and an inertia based on the weight of the shoe 92b itself act on the rear side shoe 92b. The resultant force of the differential pressure and the inertia becomes the load. The differential pressure does not changes due to the specific gravity of the shoe 92b, but the inertia changes due to the specific gravity of the shoe 92b, so the load by which the rear side shoe 92b is press-contacted against the swash plate 91 changes depending on the specific gravity of the shoe 92b. This load changes according to the rotational angle. As shown in FIG. 6, when the load becomes 0 or minus (in the rear direction) at the start of the angular range xcex1 between the top dead center T and bottom dead center U, the rear side shoe 92b separates from the swash plate 91. When the load becomes a plus one (in the forward direction) at the end of the angular range xcex1, the rear side shoe 92b strikes the swash plate 91. Here, the energy E when the shoe 92b strikes the swash plate 91 is expressed as follows when the mass of the shoe 92b is xe2x80x9cmxe2x80x9d and the speed of the shoe 92b is xe2x80x9cvxe2x80x9d:
E=(xc2xd)mv2 
Therefore, a difference arises in the energy E depending on the mass of the shoe 92b. 
Therefore, if the shoe 92b is mainly comprised of a ferrous metal having a large specific gravity such as SUJ2 of Japanese Industrial Standard (the JIS), the mass of the shoe 92b is large and wear is caused with the surface of the swash plate 91. In particular, when using a swash plate 91 comprised of a swash plate substrate formed with a coating for improving the slidability, the coating becomes easily worn.
An object of the present invention is to provide a single-headed piston type swash plate compressor which can prevent wear of the swash plate, in particular wear of the coating on the swash plate substrate, and in turn exhibit a superior durability.
According to the present invention, there is provided a single-headed piston type swash plate compressor provided with a housing internally defining and forming cylinder bores, a crank chamber, a compression chamber, a suction chamber, and a discharge chamber; a single-headed piston accommodated in each of the cylinder bores to be able to reciprocate therein and to define the compression chamber therein; a drive shaft driven by an external drive source and supported by the housing; a swash plate synchronously rotatably supported with respect to the drive shaft; and a pair of shoes provided at the front and rear of the swash plate so as to be accommodated in the piston and to drive the piston; wherein the swash plate is comprised of a swash plate substrate made of a first metal; and at least one shoe provided at a compression chamber side are mainly comprised of a second metal or resin with a smaller specific gravity than the first metal.
Preferably, each shoe is mainly comprised of the second metal or resin.
According to a second aspect of the present invention, there is provided a single-headed piston type swash plate compressor provided with a housing internally defining and forming cylinder bores, a crank chamber, a compression chamber, a suction chamber, and a discharge chamber; a single-headed piston accommodated in each of the cylinder bores to be able to reciprocate therein and to define the compression chamber therein; a drive shaft driven by an external drive source and supported by the housing; a swash plate synchronously rotatably supported with respect to the drive shaft; and a pair of shoes provided at the front and rear of the swash plate so as to be accommodated in the piston and to drive the piston; wherein the swash plate is comprised of a swash plate substrate made of a first metal and a coating formed on at least one compression chamber side of the swash plate for improving the slidability with the first metal and at least one shoes provided at the compression chamber side of the swash plate are mainly comprised of a second metal or resin with a smaller specific gravity than the first metal.
Preferably, the at least one shoe is comprised of a shoe substrate comprised of the second metal or resin and a coating formed on the surface of the shoe substrate for improving the slidability.
Preferably, the at least one shoe is impregnated by a lubricating oil.
Preferably, the at least one piston is mainly comprised of a third metal or resin having a specific gravity smaller than the first metal.
Alternatively, the at least one piston is comprised of a piston substrate comprised of the third metal or resin having a specific gravity smaller than the first metal and a coating formed on the surface of the piston substrate for improving the slidability.
Preferably, the inclination angle of the swash plate is variable with respect to the drive shaft and the pressure in the crank chamber can be adjusted by a control valve to change the inclination angle and adjust the amount of discharge.